Why does ionice(1) ban the user to set back to 'none' class?

[Zheng Liu <gnehzuil.liu@gmail.com>]

Hi Jens,

I meet a problem when I use ionice(1) to adjust a process's io priority.
I do the following operations:

$ ionice -p${pid}
none: prio 0
$ ionice -p${pid} -c2 -n4
$ ionice -p${pid}
best-effort: prio 4
$ ionice -p${pid} -c0 -n0
$ ionice -p${pid}
best-effort: prio 0

So I cannot set scheduling class back to 'none'.  If I call ioprio_set(2)
directly, it will be fine.  But if I use ionice(1), I cannot change it.  I
read the docs about ionice in [1].  I notice this code:

switch (ioprio_class) {
	case IOPRIO_CLASS_NONE:
		ioprio_class = IOPRIO_CLASS_BE;
                               ^^^^^^^^^^^^^^^^
                               *It means that we cannot set back to none.*
		break;
	case IOPRIO_CLASS_RT:
	case IOPRIO_CLASS_BE:
		break;
	case IOPRIO_CLASS_IDLE:
		ioprio = 7;
		break;
	default:
		printf("bad prio class %d\n", ioprio_class);
		return 1;
}

My question is why we need to ban the user to set back to 'none'.  Is there
some reasons?  Thank you.

[Sorry, I don't subscribe linux-doc and linux-kernel mailing list.
Please CC to me.]

1. ${linux_src}/Documentation/block/ioprio.txt.

Regards,
Zheng